Lapping method and apparatus



Jan. 14, 195 i. G. LEWIS 2,819,572

LAPPING METHOD AND APPARATUS Filed Aug. 28, 1956 INVENTOR THOMAS G. LEWIS BY g- W ATTORNEY United States Patent LAPPING METHOD AND APPARATUS Thomas G. Lewis, Wilmington, DeL, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del, a corporation of Delaware Application August 28, 1956, Serial No. 606,637

6 Claims. (Cl. 51-289) This invention relates to improvements in method and apparatus for lapping metallic articles, and particularly to a method and apparatus for the lapping of articles fabricated from non-homogeneous or inclusion-bearing metal alloys and metals to develop thereon a high quality surface finish.

Metal alloys and some metals, to varying degrees, are non-homogeneous to abrasion and therefore it is difficult to lap these materials at a substantially uniform rate necessary to develop high quality surface finishes with good dimensional control. Typical examples of such materials are the high tungsten, vanadium and chromium content alloy steels, wherein relatively rich carbide concentrations exist at the grain boundaries or at other points. These carbides are usually much harder than the metal of the grain interiors and, in the case of vanadium carbide, at least, may be of a greater hardness than aluminum oxide abrasive itself. An even more serious problem from the standpoint of lapping is that of the impurity inclusions which are found in all commercial grades of metal alloys. These inclusions may comprise small amounts of silicates, sulfides or other substances, or refractory particles picked up from the smelting furnace, which may be either harder or softer than the primary metal constituent and thus possess a different abrading characteristic. When metal alloys are lapped by the conventional methods now known to the art, inclusions invariably tear away from the surrounding metal, leaving pits behind which reduce the quality of the finish. Usually the dislodged inclusions join the abrasive and are forced against the work surface one or more times during later passes of the lapping tool, thus producing scratches which additionally mar the finish. Yet another cause of pitting is believed to be point welding which may occur between the tool and the work under the high temperatures which exist within the small area of contact. The breaking of point welds under the relative movement of the lap and the work tears small pieces of metal away, with resultant pitting, and the free metal particles intermingle with the abrasive and deface the work surface upon being carried into the contact area during subsequent operation.

U. S. Patents 2,612,736 and 2,697,900, property of the same assignee, taught processes and devices for the attainment in materials of the type hereinabove mentioned of lapped surfaces having qualities of the order of 12 microinches root-mean-square, and the instant invention constitutes an improvement over the patented subject matter by which it is possible to obtain even higher surface qualities, but especially finished work which is essentially free from blemishes caused by particles dislodged in the course of lapping operation. High quality surface finishes are essential to certain uses of metals in polymeric material film casting wheels, polishing rolls for coated fabrics and the like. Ordinarily, alloy steels, such asthe high chromium stainless steels, are utilized in the construction of such wheels and rolls and, even though premium grades of the alloys are utilized, trouble- Patented Jan. 14, 1958 some inclusions are still always present. =Even pure metals, such as electroplated chromium, for example, show non-homogeneities to abrasion, since it is very diificult to secure a high quality surface finish thereon while still retaining good dimensional control.

I have now found that it is possible to lap metal alloys and also metals to finishes having qualities of 0.3 to 0.4 microinch R. M. S. with the practical elimination of inclusion dislodgement in the course of the lapping operation by forcing a diamond dust abrasive against the work with a balsa wood lap which has been surfacetreated with resins extracted from lignum vitae wood, While maintaining a predetermined pressure of contact and a 'high relative speed of translation of the surfaces, whereby areas are abraded at substantially the same rate, but inclusions are not torn away from the matrix metal. A primary object of this invention, therefore, comprises providing a method and a device for the lapping of articles fabricated from metal alloys or metals. to exceedingly high quality finishes while, at the same time, avoiding blemishes incident to the lapping operation. Other objects include the provision of a method of lapping articles fabricated from metal alloys or metals which is rapid and economical, and which is subject to high dimensionsal control, so that the finished articles conform to very precise size standards. The manner in which these and other objects of this invention are attained will become apparent from the detailed description of this specification and the following drawings, in which:

Fig. 1 is a perspective view of an apparatus adapted to lap a work-piece of cylindrical shape according to this invention, the outline of the work in set up position being shown by broken line representation;

Fig. 2 is a rear elevational view showing a preferred disposition of lap in relationship to a work-piece of cylindrical shape;

Fig. 3 is a front elevation of one embodiment of lap according to this invention; and

Fig. 4 is a sectional view taken along the Vertical axis of the lap shown in Fig. 3.

Generally, the method of lapping according to this invention comprises forcing a diamond abrasive against the work with a balsa wood lap surface-coated with ex tracted lignum vitae resins, the invention further including the lapping device per se, which lap is utilized to contact the work with substantial pressure while being rotated with a relative surface speed sufiiciently high to attain a lapping action.

The lap of this invention may conveniently comprise a single piece of balsa wood cut transverse the trunk, the ends of the fibers presented constituting a more uniform surface than is obtained when the wood is cut with the grain. Large laps can be built up by assembling a multiplicity of pieces cut across the grain into the lap configuration desired and holding the pieces rigidly together by a suitable device such as that hereinafter described wtih reference to Figs. 3 and 4, so that a substantially continuous surface is provided for forcing the abrasive against the work. Since it is necessary to maintain high relative surface speeds during lapping, it is preferred to employ laps of annular shape in the practice of my method, for the reason that this configuration is stable and strong enough to withstand very high rotational speeds.

In Figs. 3 and 4, the balsa wood lap 9 comprises a unitary annulus cut transverse a block of the wood, so that the ends of the wood fibers make up the working face 10 which contacts the work during lapping. Since the lap is subjected to relatively high stresses during use, it is desirable to provide support by mounting the lap tightly within the annular recess 11 of the circular metal holder 12 with a light, drive fit and to cement securely the surface of the lap in contact with recess 11 to holder 12 with a suitable high strength commercially available cement. The rear face of holder 12 is provided with tapped holes 13 for the reception of bolts for attachment of the lap to a flange of a power rotating spindle. The lap shown in Figs. 3 and 4 is adapted to the lapping of cylindrical work pieces and the working face is therefore machined to a plane, smooth surface throughout. Where it is desired to lap plane surfaces, it is preferred to machine the outer edge of face 10 on a bevel and maintain holder 12 during operation at an inclination such that the beveled edge is parallel to the work surface, the work and tool being indexed laterally relative to one another to the extent necessary to lap by successive traverses the desired surface area, whereupon substantially equal areas of the lap contact the work as in the case of cylindrical work when the same loading pressures are used.

Lapping according to this invention can be carried out with conventional machine shop equipment of the type shown in Fig. 1, wherein the cylindrical work-piece 19, in broken line representation, is set up between the centers and turned by a standard grinding machine 20, such as a Brown & Sharpe, model No. 4, universal grinding machine. The lap, of the design shown in Figs. 3 and 4, is powered by a heavy duty motorized spindle 21, such as 21 Pope Machinery Corporation series P-2500 design, which is substituted for the conventional grinding wheel of the grinding machine but is made to traverse the lap lengthwise of the work piece 19 by operation of the grinding machine 20 in the usual manner. The in-feed of the lap against the work is conveniently gaged by maintaining the horsepower input to the spindle motor at a sufficient level for each of the sizes of abrasive used to maintain high quality lapping at a satisfactory rate. A power demand meter, not shown, of the type represented by the Weston model 639 industrial analyzer is connected in circuit with the motor of spindle 21 to provide the operator with an indication of the existing loading during operation. It will be understood that the particular apparatus hereinabove described constitutes but one arrangement of many which can be employed for the purpose, and that numerous other arrangements known to those skilled in the art may be used, depending on the size and shape of the work to be lapped and the practicable speeds at which the work or the lap can be turned.

In the lapping of cylindrical work, good results are obtained when the surface of the lap is disposed substantially tangent to the surface of the Work, as indicated in Figs. 1 and 2. The center line of the lap-powering spindle may be coplanar with the longitudinal axis of the work or may be disposed thereabove; however, it is preferred to locate the lap as shown in Fig. 2, wherein the angle between the median line of contact of the lap with the work and the vertical is approximately degs. With this disposition, the right-hand area of contact of the lap passes the work at an inclination of 45 degs. to the right, while the left-hand area of contact passes the work at an inclination of 45 degs. to the left, thus providing a broken pattern which is desirable in lapping regardless of the particular method involved. Equally good results are obtained when the lap is turned in either a clockwise or counterclockwise direction as viewed in Fig. 2. In the practice of my method I have found that it is necessary to maintain a relative speed of translation between the contacting surfaces of the lap and the work of the order of about 5,000 ft./min., and preferably 7,500-l0,000 ftJmin.

It is essential to the purposes of this invention to ernploy a balsa wood lap in conjunction with the diamond dust abrasive which has been surface-coated with extracted lignum vitae resins. The exact functioning of the surface-coated balsa wood is not understood with certainty; however, it appears thatthe peculiar nature of the balsa wood and the thin layer of resins interposed between the lap face and the work combine to afford a resilient, diamond-retentive lapping surface which is responsible for the very high surface qualities which are obtained, while at the same time exerting a minimum of drag on the inclusions which would tend to dislodge them and thereby blemish the surface of the work. It is preferred to utilize a balsa wood which has a softness such that it will not hold the diamond dust against the work with suflicient force to cause scratches, while still possessing enough retentivity for the diamond to affect a certain amount of lapping. I have found that there is a limiting softness for balsa wood which renders it useless for the purposes contemplated, which can be determined by testing the wood in a conventional Rockwell hardness tester with a diameter tool steel ball under a loading of 60 kgs. The test is conducted on the wood in an end grain direction and measurement of the diameter of the crater left by the steel ball on cessation of penetration, which usually occurs within about 20 secs., affords a convenient index of the hardness. Indentations of diameters in excess of about are an indication that the wood is too soft for use as a lap. Insofar as my experiments have revealed, there is apparently no upper limit on the hardness of balsa which affects lapping action.

The lignum vitae resin coating is applied to the balsa wood to give a continuous coating which, in appearance, constitutes an unbroken, relatively hard surface of greenish cast which resembles a heavy varnish coat, but apparently has unique properties which distinguish it from all other known materials as a lapping aid. The function of the resin coating appears to be that of a retainer for the diamond abrasive; however, the resin incidentally seals the lignum vitae lap against the penetration of water which must be sprayed on the work during lapping to effect cooling, and there are probably other cons1derations too complex for evaluation in the light of present knowledge explainable of the results obtained In any event, very even, high-grade lapping is obtained with the lap of this invention without objectionable differential heat generation, by which is meant the generation of heat within local areas which eventually causes unequal expansion of the work-piece resulting in eccentric rotation or whipping sufficiently serious to cause the loss of dimensional control.

The lignum vitae resins are obtained by subdividing lignum vitae wood, such as by use of a lathe or a jointer adjusted to cut away the surface of the wood to depths of 0005-0010", to thereby obtain particles of the order of size, which afford a large surface for contact with the solvent. In a typical extraction, loose lignum vitae powder Weighing about 400 gms./liter was put into a 5,000 cc. Florence flask which was then filled with solvent. A mixed solvent comprising 50% by volume of benzene and 50% by volume methyl alcohol was employed in the ratio of about 985 gms. of solvent/liter of loose lignum vitae wood powder. The best solvent known to me consists of a mixture of benzene and methyl alcohol, which apparently is effective in extracting certain lignum vitae resin components necessary to successful lapping which are either not removed by other solvents or are affected deleteriously, thereby causing the test results obtained with other solvent-extracted resin fractions to be noticeably inferior. In this connection, the next solvent in order of preference was found to be ethyl ether, which extracted resin of about the same lapping effectiveness, but with a much lower yield, besides which the ethyl ether is objectionable from the standpoint of its inflammability. Other solvents, such as carbon tetrachloride, methyl ethyl ketone, acetone, and certain commercially available chlorinated hydrocarbons, were about equal in their extraction power but did not yield quality lapping comparable with the benzene-methyl alcohol extract.

The lignum vitae wood powder was solvent-extracted in accordance with conventional practice using a Soxhlet extraction apparatus wherein the solvent was refluxed for 2-3 hours, thereby putting the resins into solution. The extraction resulted in the removal of from about 18 to 23% by weight of the lignum vitae wood on an air dry basis. The liquid extract was removed from the wood powder by filtration, giving about 400 gms. of concentrate as yield. The extracted lignum vitae wood powder was washed with about 4,000 cc. of mixed benzene-methyl alcohol solvent to remove surface material, the wash being combined with the concentrate. The mixture was then evaporated to about the original weight of 400 gms., which thickened the extract to the consistency of a heavy varnish of blackish-brown color. In this condition, the extract was stable in storage over a period of months.

The lignum vitae extract, diluted if desired with enough mixed benzene-methyl alcohol solvent to facilitate brush application, was applied to the balsa wood in 3-5 coats with drying after each coat. The characteristic cellular structure of balsa wood limited penetration of the resin to a depth of about 0.005-0.010. In a test in which the balsa wood was first placed in a vacuum and the resin thereafter added, followed by the application of superatmospheric pressure, it was found that resin penetration was not thereby improved, even though the capillaries in the wood were filled with the resin, but instead that the wood was compressed to an increased density by collapse of the individual cells of the structure, which was not desirable.

The edges of the lap and holder are preferably sealed against leakage of water at these points by applying the resin liberally in this region to build up an impervious sealing fillet. Resin drying can be accelerated by the application of heat, such as through the use of infrared lamps, the end of the drying process being signaled by cessation of bubbling of the solvent in the course of vaporization. The drying temperature should be kept to below about 400 F. to avoid charring which is indicated by a gradual blackening of the surface, overheating giving a brittle resin surface of reduced polishing effectiveness.

Lapping according to this invention has proved particularly etfective for the high quality finishing of hard alloy steels, of which the loadings utilized for 440-A and Armco l7-4-PH, reported in detail hereinafter, constitute a guide which, supplemented by observations in the course of lapping other materials, will enable others to practice my method:

A 36" diameter roll fabricated from high chromium steel, typified by the analysis of A181 440-A and Armco 174PH, having a face width of 22 and a wall thickness of was mounted on a mandrel and lapped according to this invention in the following manner, using the apparatus shown in Figs. 1-4. The lap comprised a 1 thick segmental balsa wood ring of 6%" inside diameter and 8%" outside diameter, thus providing a working face of 1 width.

The as-received state of the shell was that it was surface ground to a quality of about 2 microinches R. M. S., the surface, however, displaying occasional surface defects due to dislodgment of inclusions or scratches caused by such dislodgment. The shell was lapped in four separate steps as hereinafter described, turning the work at a speed of 2 R. P. M. with a lap spindle speed of 3600 R. P. M. and a traverse travel lengthwise of the work of 2"/min., giving a relative surface speed between the lap and the work of about 7600'lmin. In all cases, the diamond dust abrasive was utilized as a suspension in olive oil. To facilitate the maintenance of temperature con- 6 trol during lapping, the mandrel supporting the shell was drilled at two points adjacent the ends of the shell and provided with rotary joints for the introduction and removal of cooling Water, which was continuously circulated through the interior of the shell during the lap ping cycle.

The rate of metal removal in the course of lapping is a function of horsepower loading applied to the lap, all other conditions remaining constant. The greater the horsepower loading, the higher the rate of metal removal; however, the greater the horsepower loading the greater is the possibility that inclusions will become dislodged and cause marring of the metal surface. As a consequence, the operator must strike a compromise between lapping speed and the final quality of finish desired. After a few trials, one can readily determine lapping rate as a function of horsepower loading by the discoloration of the olive oil-diamond dust slurry caused by the concentration of removed metal in particulate form, which is built up in the slurry in a given time. The darker the slurry becomes within a certain time of operation, the greater is the lapping rate and, at least at the higher end of the pressure range when the balsa wood gives evidence of being permanently compressed under the loading, the greater is the likelihood that inclusions will dislodge and probably scratch the metal surface.

It has been my experience that there is a relatively wide latitude of choice as regards lap loadings and that, with ordinary care in visual observations, there is little danger that the work will be spoiled. Erosion of the lap necessitated redressing every 24- hours, which was done by cutting the face of the lap away to a depth of about with a sharp pointed lathe tool and then recoating with lignurn vitae resin extract in the identical manner hereinbefore described.

Step 1 Lapping was initiated with 1,200 grit size diamond dust in a concentration of about 20 carats/1000 cc. olive oil, equivalent to a concentration of about 1 carat/sq. ft. of work surface. The time of lapping was 40 hours with a loading of 1.3 H. P., as calculated from the reading of a demand meter in circuit with the spindle motor, and the final quality of the surface obtained was 1.0 microinch R. M. S. At the conclusion of the lapping step, the lap was thoroughly washed with carbon tetrachloride and the outer face cut away to a depth of about after which the face was coated with fresh lignum vitae resin preparatory to carrying out Step 2.

Step 2 Diamond dust abrasive of 4,000 grit size was utilized in a concentration of 17 carats/ 1000 cc. olive oil, equivalent to a concentration of 0.88 carat/ sq. ft. of working surface using a loading of 1.3 H. P. with a total lapping time of 40 hours. The final quality of surface obtained was about microinch R. M. 8., after which the lap was renovated by cutting away the face and painting with fresh lignum vitate resin extract before going on with the lapping.

Step 3 The abrasive employed was of 8,000 grit size diamond and a concentration of 15 carats/1000 cc. of olive oil, giving a concentration of about 0.75 carat/ sq. ft., with a loading of 1.0 H. P., the total lapping time being 40 hours. The final quality of surface obtained was about /2 microinch R. M. S., and the lap was renovated as described at the conclusion of Steps 1 and 2.

Step 4 A 14,500 grit size of diamond dust was. used in this finished step with a concentration of 10 carats/1000 cc. olive oil, equivalent to 0.5 carat/sq. ft., at a loading of 0.8 H. P., giving a quality of surface of 0.3-0.4 microinch, at which the lapping operation was terminated.

In summary, the lapping process and device of this invention have been found to produce a finer over-all finish than any other procedures known to me by an action which is believed to be attributable to the reduced tendency toward inclusion dislodgment. The lignum vitae resin coating is apparently unique in its contribution to this objective, because resins extracted from numerous other wood species, as well as lignin resin itself, were found to be completely ineffective as well as being generally water soluble, which is undesirable Where water is encountered. as accidentally occurs where water cooling of the work is resorted to. It was also found that the lapping process of this invention generated less heat than is generated with previous lapping techniques, resulting in a reduced Work distortion and corresponding benefits in the maintenance of dimensional stability.

From the foregoing, it will be understood that my invention may be modified in numerous ways obvious to those skilled in the art without departing from the essential spirit, wherefor it is intended to be limited only by the scope of the following claims.

What is claimed is:

1. A method for the lapping of an article fabricated from a metal alloy or metal comprising forcing a diamond dust abrasive against said article in the presence of a liquid with a balsa wood lap, said lap having a hardness greater than that evidenced by a diameter of indentation measuring in excess of about for a /8 diameter tool steel ball pressed against said balsa wood with a loading of 60 kgs. in an end grain direction, the work-contacting face of said balsa wood lap being coated with extracted lignum vitae wood resin, while maintaining a relative speed of translation between the surfaces of said lap and said article above about 5,000'/min. and a loading pressure sufficient to cause removal of metal from the surface of said article by lapping action.

2. A method for the lapping of an article fabricated from a metal alloy or metal according to claim 1 wherein the work-contacting face of said balsa wood iscoated with lignum vitae resin extracted with a solvent consisting essentially of benzene and methyl alcohol in mixture.

3. A method for the lapping of an article fabricated from a metal alloy or metal in two separate contactings of the lap with said article wherein the direction of translation of the lap with respect to the surface of said article during one of said contactings is maintained substantially normal to the direction of translation of the lap with respect to the surface of said article during the other said contacting, each of said contactings comprising forcing a diamond dust abrasive against said article in the presence of a liquid with a balsa wood lap, said lap having a hardness greater than that evidenced by a diameter of indentation measuring in excess of about for a diameter tool steel ball pressed against said balsa wood with a loading of 60 kgs. in an end grain direction, the work-contacting face of said balsa wood lap being coated with lignum vitae resin extracted with a benzenemethyl alcohol mixed solvent, While maintaining a relative speed of translation between the surfaces of said lap and said article above about 5,000'/min. and a loading pressure suflicient to cause removal of metal on the surface of said article by lapping action.

4. A method for the lapping of an article fabricated from a metal alloy or metal according to claim 3 wherein the liquid comprises olive oil.

5. A method for the lapping of an article fabricated from a metal alloy or metal in a succession of steps employing progressively finer graded particle sizes of diamond dust abrasive comprising forcing each of said sizes in turn against said article in the presence of a liquid with a balsa Wood lap, said lap having a hardness greater than that evidenced by a diameter of indentation measuring in excess of about W for a diameter tool steel ball pressed against said balsa wood with a loading of 60 kgs. in. an end grain direction, the work-contacting face of said balsa Wood lap being coated with lignum vitae resin extracted with a benzene-methyl alcohol mixed solvent, while maintaining a relative speed of translation between the surfaces of said lap and said article above about 5,000'/min. and a loading pressure suificient to cause removal of metal on the surface of said article by lapping action, and cleaning the surface of said article after each step before beginning the next succeeding step.

6. A lap for lapping the surface of an article fabricated from a metal alloy or metal comprising balsa wood cut transverse the grain having the work-contacting surface of said wood coated with lignum vitae resin extracted with a mixed benzene-methyl alcohol solvent.

References Cited in the file of this patent UNITED STATES PATENTS 2,380,275 Turner et al. July 10, 1945 2,471,727 Creamer May 31, 1949 2,697,900 Lewis Dec. 28 ,1954

U. S. DEPARTMENT OF COMMERCE PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,819,572 January 14, 1958 Thomas G. Lewis It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 58, for "Vite-be" read --vitae--; lines '71 and '72, for finished read --finishing--.

Signed and sealed this 25th day of February 1958,

(SEAL) Atteet:

KARL AXLINE ROBERT c. WATSON Attesting Officer Conmissioner of Patents 

1. A METHOD FOR THE LAPPING OF AN ARTICLE FABRICATED FROM A METAL ALLOY OR METAL COMPRISING FORCING A DIAMOND DUST ABRASIVE AGAINST SAID ARTICLE IN THE PRESENCE OF A LIQUID WITH A BALSA WOOD LAP, SAID LAP HAVING A HARDNESS GREATER THAN THAT EVIDENCED BY A DIAMETER OF INDENTATION MEASURING IN EXCESS OF ABOUT 9/32" FOR A 5/8" DIAMETER TOOL STEEL BALL PRESSED AGAINST SAID BALSA WOOD WITH A LOADING OF 60 KGS. IN AN END GRAIN DIRECTION, THE WORK-CONTACTING FACE OR SAID BALSA WOOD LAP BEING COATED WITH EXTRACTED LIGNUM VITAE WOOD RESIN, WHILE MAINTAINING A RELATIVE SPEED OF TRANSLATION BETWEEN THE SURFACES OF SAID LAP AND SAID ARTICLE ABOVE ABOUT 5,000''/MIN. AND A LOADING PRESSURE SUFFICIENT TO CAUSE REMOVAL OF METAL FROM THE SURFACE OF SAID ARTICLE BY LAPPING ACTION. 